2 DEPARTMENT BULLETIN 1110. 
box, for the purpose of cleaning the structure of sand and silt. This 
manipulation makes it difficult to restore the crest of the weir to its 
former elevation and has led to the practice of measuring the head, 
for free-flow weirs, on the crest instead of at the standard gauge 
point, or place for measuring the head. If, however, the weir be- 
comes submerged, as frequently happens when large quantities are 
measured over low weirs, it 1s necessary to measure both the up- 
stream and the downstream heads in order to find the discharge. 
In this case two gauge points, or places for measuring the head, 
Fic. 1.—Standard plan of farmer’s short-box measuring flume. 
must be used, one above and one below the weir bulkhead. Both 
points should be at the same elevation. The floor of the weir box, 
which is level, may be used in an emergency, but under conditions 
in which submergence frequently occurs points located at the eleva- 
tion of the weir crest are more convenient. 
METHOD OF CALIBRATION. 
For the purpose of simplicity in making the calibrations of the 
various sizes of flumes and heights of weirs, and in order to make the 
results comparable, it was necessary to adopt a standard design for 
